Gsm architecture

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Gsm architecture

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Gsm architecture

  1. 1. By Mustahid ali 1
  2. 2.  GSM defines a standard for a Public Land Mobile Network (PLMN)  GSM is a globally accepted standard for digital cellular communication    Because GSM provides a common standard, cellular subscribers can use their telephones over the entire GSM service area which includes all the countries around the world where GSM system is used  Three bands of operation : GSM900 GSM1800 GSM1900 (used in USA)  GSM900 has its up-link and down-link frequencies in the 900MHz range  Up-link frequency band is from 890MHz to 915MHz (25MHz)  Down-link frequency band is from 935MHz to 960MHz (25MHz)   The 25MHz up-link and down-link frequency band is divided into 125 RF Channels or radio carriers with inter-carrier spacing being 200KHz 2
  3. 3. NK PLI U K LIN N OW D 200KHz 890MHz 915MHz 0 124 BW = 25MHz 125 Channels of 200KHz BW 935MHz 960MHz 0 124 BW = 25MHz 125 Channels of 200KHz BW 3
  4. 4.    f The access method in GSM is both FDMA & TDMA FDMA (Frequency Division Multiple Access): achieved by dividing the available radio spectrum into 125 sub-channels each of 200KHz bandwidth so that multiple users can access the system at the same time TDMA (Time Division Multiple Access) : each carrier is divided into 8 Time Slots so that each Frequency Channel is shared by 8 users at a time on time sharing basis Carrier-124 Carrier-1 Carrier-0 TS0 TS1 TS7 t One TDMA frame 4
  5. 5. ARFCN (Absolute Radio Frequency Channel Number) Absolute Radio Frequency Channel Number (ARFCN) is a serial Number of the Uplink and Down-link Frequencies, 0 through 124  One ARFCN corresponds to an Up-link frequency and a Down-link frequency 45MHz apart  BSNL has been allotted 31 ARFCNs  45MHz 890MHz 915MHz 0 124 ARFCN 0 ARFCN 1 935MHz 0 ARFCN 124 960MHz 124 5
  6. 6. GSM Time Division Multiple Access Frame and Physical Channels Physical Channels are the Time Slots in TDMA Frame Time-slot TDMA frame TDMA frame TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 (frames repeat continuously) Time 0 4.615 ms 9.23 ms Physical channel # 2 = recurrence of time-slot # 2 6
  7. 7.   One Time Slot has a duration of 0.577mS (148 bits) 8 timeslots (8 x 0.577 = 4.615 ms) form a TDMA frame 7
  8. 8. GSM NETWORK ARCHITECTURE – Mobile Switching Centre MSC VLR – Visitor Location Register HLR – Home Location Register AUC – Authentication Centre EIR – Equipment Identity Register AUC BSC – Base Station Controller EIR BTS – Base Trnsceiver Station PLMN – Public Land Mobile Network (other GSM operators) PSTN – Public Switched Telephony Network (Land Line N/W) Billing ISDN – Integrated Services Digital Network (Data Network, Fax, etc.) HLR SMSC IN IN – Intelligent N/W Server (Prepaid and Virtual Private N/W) SMSC – Short Message Service Centre PLMN VLR PSTN MSC ISDN SGSN – Service GPRS Support Node GPRS – General Packet Radio Services MS – Mobile Station BSC BTS MS SGSN Cells 8
  9. 9. GSM NETWORK ELEMENTS MSC (Mobile Switching Centre) The MSC is the primary node of the GSM cellular system          Performs Call Processing like call set up, switching of the call, call termination and charging Provides interface of the GSM network to PSTN and other PLMNs Routes calls between the GSM N/W and PSTN and other PLMNs Interrogates HLR for MSRN in order to route a call to a Mobile Station Inter-BSC Handover Paging of the MS for an Incoming Call, SMS Generates Billing data and routes Call Detail records (CDRs) to the Billing server Monitors Traffic and Load on the System and different Routes Generates Reports on Network performance, etc. 9
  10. 10. GSM NETWORK ELEMENTS HLR (Home Location Register) HLR is a permanent database of the Subscriber Services    Stores user data of all Subscribers related to the GMSC ◦ International Mobile Subscriber Identity (IMSI) ◦ Users telephone number (MSISDN) ◦ Subscription information and services (like STD, ISD, Call Forwarding, Call Waiting, SMMO, SMMT, Roaming Subscription Information, etc.) ◦ Current VLR address and LAI (current location of the MS) Referred to for an Incoming Call for MSRN (Mobile Subscribers Roaming Number) A subscriber’s data are deleted from the HLR database only when his subscription is ended 10
  11. 11. GSM NETWORK ELEMENTS VLR (Visitor Location Register) VLR is a temporary database of the subscribers currently present in the MSC/VLR service area          When a subscriber enters the MSC/VLR area a copy of his Subscription details is copied to the VLR database from his home HLR Contains Subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR Identity of Mobile Subscriber (MSISDN, IMSI, etc) Copy of subscriber data from HLR (so that each time the subscriber establishes a call or has an incoming call, or for SMS, the HLR need not be queried which would increase the Call Processing Time and Signaling Load) Generates and allocates a Temporary Mobile Subscriber Identity(TMSI) Contains Location Area Identity (LAI) – a group of cells Updates the LAI when a subscriber changes location Contains the IMEI of the MS The subscribers database form the VLR is deleted when he moves to another MSC/VLR area 11
  12. 12. GSM NETWORK ELEMENTS AUC (Authentication Centre) Authenticates the subscriber’s IMSI to receive service in the MSC/VLR area (own subscribers or from MSCs with whom we have Roaming agreement, like BSNL allover India, MTNL Delhi and Mumbai. And not subscribers from Private operators with whom we don’t have Roaming agreement)  At subscription time, each subscriber (SIM) is assigned an Authentication Key (Ki). Ki is stored in the AUC along with the subscriber’s IMSI. The same Ki and IMSI are also stored in the SIM. In an AUC the following steps are carried out :       A non-predictable random number, RAND, is generated RAND and Ki are used to calculate SRES (Signed Response) and Cipher Key (Kc), using two different algorithms, A3 and A8 respectively RAND, SRES and Kc are delivered together to the HLR as a triplet The MSC/VLR transmits the RAND to the MS The MS computes the Kc and SRES using RAND and the authentication key (Ki) The SRES is sent back to MSC/VLR, which performs authentication, by checking whether the SRES from the MS and the SRES from the AUC match. If so, the subscriber is permitted to use the network. If not, the subscriber is barred from network access. 12
  13. 13. GSM NETWORK ELEMENTS EIR (Equipment Identity Register) EIR The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.  EIR is a database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its International Mobile Equipment Identity(IMEI)  EIR has three databases: ◦ White list - For all known, good IMEIs ◦ Black list - For all bad or stolen handsets ◦ Grey list - For handsets/IMEI’s that are faulty or non-approved mobile equipment 13
  14. 14. GSM NETWORK ELEMENTS BSC (Base Station Controller) The BSC is a high capacity switch that manages all the radio-related functions of a GSM network. It also provides physical links between the MSC and BTS.         Monitors and controls several BTSs Manages channel allocation on the radio interface during a call process Alarm Handling from the external interfaces and BTSs Performs inter-cell Call Handover Interface to OMC (Operation & Maintenance Centre) for BSS (Base Station system) Management It contains the Cell configuration data (like Cell Identity, Frequencies/BCCH list, Handover parameters, Neighbour cells, BTS power data, etc.) The BTS collects data on signal strength and quality of the neighbouring cells. The BSC uses these data to allocate a channel during call setup or call handover Several BSCs can be controlled by an MSC 14
  15. 15. GSM NETWORK ELEMENTS BTS (Base Transceiver Station) The BTS is the radio equipment (transceivers and antennas) needed to serve each cell in the network. A group of BTSs are controlled by a BSC.      The BTS's main function is to provide connection with the MSs over the air interface. Consists of one or more radio terminals (called TRU – Transceiver Unit) for transmission and reception Each Radio terminal/TRU handles an RF Channel/ARFCN Receiption of channel requests from MSs (during a call setup, handover, etc) The BTS is responsible for the processing of signals before transmission and after reception. This includes: ◦ Ciphering using the ciphering key (Kc) ◦ Modulation and Demodulation (GSM uses GMSK-Gaussian Minimum Shift Keying), etc. 15
  16. 16. GSM NETWORK ELEMENTS The Cell The cell is basic unit of the Cellular System. It is the Geographical area covered a BTS.  Each cell is assigned a unique number called Cell Global Identity(CGI). 404-77-80-10001 Mobile Country Code Mobile Network Code (404 for India) BSC Identity Cell Identity (77 for BSNL NE) CGI 16
  17. 17. GSM NETWORK ELEMENTS Why Cellular structure The Need of the Cell  Cellular structure increases capacity : In GSM Eight Subscribers can share an ARFCN using TDMA. And we have only 31 ARFCNs. So if we had only one BTS covering the whole service area, we would end up with only 31 x 8 = 248 Subscribers talking simultaneously. So the whole Service area is divided into many Cells covering only a small area. And cells having similar frequencies are placed a little distance away so that their frequencies don’t interfere.  Optimum Spectrum Usage : with only limited number of ARFCNs avilable the option is to re-use the frequencies a little distance away. This way we can have hundreds of cells wisely placed so that their frequencies don’t interfere (using only 31 frequencies)  Less transmission power needed 17
  18. 18.  The capacity of the Cell is limited by the number of TRUs of the BTS. If a Cell has 4 TRUs (each TRU handles one ARFCN), the capacity of the Cell becomes : 4TRU x 8TS/ARFCN = 32  Cell size ranges from some 100 m in cities to, e.g., 35 km on the country side 7 6 2 1 5 7 6 Cluster-1 1 5 3 4 Cluster-1 2 3 4 18
  19. 19.   Portable Card with memory (ROM-6KB to 16KB-has A3/A8 algorithms, RAM- 128KB TO 256KB, EEPROM- 3KB to 8KB ) It contains: ◦ International Mobile Subscriber Identity (IMSI) ◦ Personal Identification Number (PIN) ◦ Pin Unlock Key (PUK) ◦ Authentication Key (Ki) ◦ Location Area Identity (LAI) 19
  20. 20. LAI-2 LAI-1 LAI-3 20
  21. 21. Mobile Station ISDN Number (MSISDN) 91 94 36102025 Country Code National Destination Code 91 for India 94 for BSNL Subscriber Number 36102025 for a subscriber Number of HLR 36 for BSNL NE   MSISDN uniquely identifies a subscriber in the GSM Network and it is the number used for dialing a Subscriber. The HLR and VLR stores the MSISDN Number. 21
  22. 22. International Mobile Subscriber Identity (IMSI) 404 77 1210002025 Mobile Country Code Mobile Network Code 404 for India Mobile Station Identification Number 77 for BSNL NE      IMSI is a unique identity allocated to each subscriber. IMSI is used by the system. All network-related subscriber information is connected to the IMSI. The IMSI is stored in the SIM, in the HLR and VLR. IMSI has a maximum length of 15 digits. 22
  23. 23. Mobile Station Roaming Number (MSRN)       The MSRN is a number temporarily allocated to a Subscriber for routing a Call to him. MSRN is allocated by the current VLR the Subscriber is in. After the Call is established the MSRN is released, and it can now be allotted to another MS for an Incoming Call Setup. The HLR stores address of the current VLR where the subscriber is located. When a call is made to a mobile subscriber , the HLR requests the current MSC/VLR to provide an MSRN as a temporary routing number for the called subscriber. Upon reception of the MSRN, the HLR sends it to the MSC which can now use this number to route the call to the MSC/VLR where the called subscriber is currently registered. MSRN has the same format as of MSISDN. In an MSC/VLR some thousands of Numbers are reserved to be used as MSRN during Call Setup. These numbers are not allotted as MSISDN to Subscribers. 23
  24. 24. International Mobile Equipment Identity (IMEI)    The IMEI is a uniquely identity of a Mobile Equipment. The IMEI can be seen by pressing *#06# The IMEI consists of the following parts: IMEI = TAC + FAC + SNR + SVN ◦ TAC : Type Approval Code - determined by a central GSM body (6 digits) ◦ FAC : Final Assembly Code - identifies the manufacturer (2 digits) ◦ SNR : Serial Number - an individual serial number of six digits uniquely identifies all equipment within each TAC and FAC (6digits) ◦ SVN : Software Version Number - allows the manufacturer to identify different versions of a given type of approved mobile (2 digits) 24
  25. 25. Thank You 25

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